1. Field of the Invention
The disclosure herein relates generally to the field of oil and gas production. More specifically, the present disclosure relates to a method and apparatus for aligning perforating charges. Yet more specifically, the present disclosure concerns a method and apparatus for confirming that a perforating charge is aligned in a certain direction prior to detonating the perforating charge.
2. Description of Related Art
Perforating systems are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically completed by coaxially inserting a pipe or casing into the wellbore, and the casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore. As is known, hydrocarbon-bearing strata, such as reservoirs, exist within these formations. The wellbores typically intersect these reservoirs.
Perforating systems typically comprise one or more perforating guns strung together, these strings of guns can sometimes surpass a thousand feet of perforating length. Included with the perforating guns are shaped charges that typically include a charge case, a liner, and a quantity of high explosive inserted between the liner and the charge case. When the high explosive is detonated, the force of the detonation collapses the liner and ejects it from one end of the charge at very high velocity in a pattern called a “jet”. The jet penetrates the casing, the cement and a quantity of the formation thereby forming a perforation in the formation that enables fluid communication between the wellbore and its surrounding formation.
FIG. 1 is a side partial cutaway view of a perforating system 5 comprising a perforating string 7 suspended within a wellbore 25. The perforating string 7 comprises a series of perforating guns 13 axially connected to one another. Tubing 9 is shown attached to the perforating string 7, the tubing 9 comprises a raising/lowering means and can also facilitate communication between the perforating string 7 and a surface truck 11. Using tubing for a raising/lowering means enables system deployment within all forms of deviated wellbores as well as horizontal wells. Optionally, the raising/lowering means may comprise a wireline or slickline, a tractor may be employed for wireline use within deviated or horizontal wells. The surface truck 11 typically includes a winch type device for perforating string 7 disposal and retrieval. Also included within the surface truck 11 is an interface enabling surface personnel to transmit commands and receive data to and from the perforating string 7. The communicated data between the surface and the string 7 is generally provided via the wireline 9. The perforating string 7 of FIG. 1 is shown disposed in a deviated portion of the wellbore 25. For the purposes of illustration, perforations 21 are shown that extend from the wellbore 25 into the surrounding formation 19.
The shaped charges should be aimed in a particular direction for maximum penetration of a hydrocarbon producing zone, for avoiding hardware attached to the casing, sand control or to avoid shooting into zones having either water, gas, or both. Aiming of the shaped charges can be accomplished by aligning the perforating gun 13 in a particular orientation. One manner of accomplishing perforating gun orientation is to asymmetrically load a perforating gun so that the loading will cause the gun to rotate into the desired or designed orientation. With regard now to FIG. 2, one manner of loading is shown that comprises including a weight 14 in the gun, where the weight runs along a portion of its inner housing or attached to the shape charge tube carrier. In some instances however, in spite of the asymmetric loading the guns may stick within the casing and not freely rotate into the desired orientation. With reference now to FIG. 2 again, the shaped charge 27 should be aligned with the line “L”, but instead points an angular distance θ away from line “L” to produce the perforation 21.